The invention pertains to a method for limiting the starting current of a step-up-converter.
A method for limiting the starting current of a step-up-converter is generally known from DE 102 55 431 A1. In this known method, the starting current is continuously measured and the step-up-converter is disconnected from a DC voltage source supplying an input voltage of the step-up-converter as soon as the starting current reaches a critical threshold value.
This disclosure teaches a method for limiting the starting current of a step-up-converter to a permissible value with little effort.
According to this disclosure, the output voltage of the voltage converter is monitored and the step-up-converter is temporarily disconnected from the DC voltage source as soon as the output voltage has increased by a predefined value since the last application of the input voltage supplied by the DC voltage source. The step-up-converter is only reconnected to the DC voltage source after a predefined time period, in which the coil of the step-up-converter discharges. As soon as the output voltage has once again increased by the predefined value since the reconnection to the DC voltage source, the DC voltage source is once again disconnected from the step-up-converter such that the coil of the step-up-converter can discharge again. The output voltage of the step-up-converter therefore is incrementally increased by repeatedly switching on and off a power switch, by means of which the step-up-converter is connected to the DC voltage source. This incremental increase of the output voltage is carried out until the output voltage has reached a predefined final value. Once the final value is reached, the power switch can remain closed as long as desired because alarmingly high currents are no longer expected due to the fact that all capacitors of the step-up-converter already have been largely charged.
The predefined final value preferably lies below the input voltage. For example, the final value may amount to three fifths of the input voltage supplied by the DC voltage source or more. The final value is dependent on the system consisting of cable harness, capacitance and permissible maximum current and therefore cannot be generally defined. In many applications, the final value amounts to three fourths of the value of the input voltage supplied by the DC voltage source.
The monitoring of the output voltage of the step-up-converter can be realized with significantly less effort than the direct monitoring of the starting current, in which case a current measurement would be required. Since the increase of the input voltage takes place incrementally, however, a rise of the starting current to problematic values can be prevented just as reliably.
The number of steps, in which the output voltage should be increased to the final value, depends on the degree, to which the starting current should be limited. As a rule, it is advantageous if the threshold value, by which the output voltage is increased in each step, amounts to no more than one tenth of the input voltage supplied by the DC voltage source. However, the threshold value may also be much lower and merely amount, for example, to 5% of the input voltage supplied by the DC voltage source or less.
According to this disclosure, the time period between disconnecting and reconnecting the input voltage can largely be chosen arbitrarily. However, the time period should be sufficiently long for allowing the coil of the step-up-converter to discharge, i.e., for at least largely reducing the current flowing therein. Good results can be achieved if the time period is so long that the charging current of the capacitor of the step-up-converter has settled to less than one tenth, preferably to no more than 5%, of a previously reached maximum value in the closed state of the power switch.